I. Field of the Invention
The present invention relates to a clamp force control on an injection molding machine, and more particularly, the invention provides an automatic process for providing a more accurate clamp force control.
II. Description of the Related Art
Various types of mold clamping arrangements have been developed to hold a pair of mold members together while molten plastics material is injected into a mold cavity defined by the closed mold members. Mold clamping forces on injection molding machines range from tens of tons to thousands of tons. Such clamping forces are necessary to prevent the internal forces within the mold from forcing the mold members apart. Too small a clamping force can result in either a thin, external tab or protrusion of plastic material on the molded article, commonly referred to as "flash," or an incomplete filling of the mold cavity as a result of excessive separation of the mold members during injection. In either case, the resulting molded part is unacceptable.
Although it would be possible to avoid the mold separation problem by consistently imposing the maximum mold clamping force, repeated operation with unnecessarily high clamping loads can result in damage to the molds and is therefore undesirable. Accordingly, the externally applied mold clamping force should bear some close relationship with the actual internal mold separation force generated within the mold cavity, and therefore, control over the mold clamping force is highly desirable.
One system for providing an automatic clamp force control is disclosed in U.S. patent application Ser. No. 07/524,312 filed on May 17, 1990 and assigned to the same assignee as the present invention. The disclosed force control provides an automatic adjustment of the die height platen and toggle mechanism in response to a desired clamping force. Clamp force control is accomplished by controlling the toggle position at which the molds come into initial contact so that the desired clamp force is attained upon the toggle reaching its fully extended position.
As part of a normal clamp closing cycle, not only is a desired final clamping force defined, but an initial clamping force or mold protect force is also defined by the machine user. The mold protect force is set to be a relatively small value such that if there is any unanticipated contact between the molds, and the clamping forces build up to the mold protect force, that force may be detected, and the clamp closing cycle terminated prior to any mold damage.
The mold protect force is typically a function of the toggle mechanism design, mold weight, tooling weight, lubrication and frictional forces within the clamp and other variables that exist from one mold and machine to the other. An ideal mold protect force is represented by the threshold forces required to overcome the gravitational and frictional forces opposing motion of the mold. Therefore, such ideal force is just large enough to permit the toggle mechanism to move the mold but small enough to cause the toggle mechanism to stall out in the event of any undesirable interference, e.g., from a molded part, an unretracted ejector, etc.. Mold protect force is typically set to a value slightly greater than the ideal value by a machine operator. To set mold protect, starting with the clamp open, the operator will typically establish successively greater mold protect values until motion of the clamp is detected. That value or a slightly greater value is then selected as the mold protect force.
Any force exerted by the clamping mechanism when the molds are in contact including the mold protect force will result in a tensional force on the tie bars connecting the stationary platen and die height platen which causes the tie bars to stretch. Therefore, if the final clamping force is to be controlled by regulating toggle position upon mold touch, and further, if different mold protect forces result in additional magnitudes of tie bar stretch, then a failure to modify toggle position as a function of those magnitudes of tie bar stretch will create a less accurate control of clamping force.